This invention pertains to a method for predicting the dynamic parameters of fluids in a subterranean reservoir.
In the search for subterranean fluids (typically natural gas and/or petroleum), classical methods have used seismic imaging to generate an image of a subterranean region of exploration. Modem three dimensional (3D) seismic imaging has aided a great deal in developing credible models of the region of interest. Information regarding travel time of seismic signals can be used to estimate velocities of various subsurface layers, which provides information indicative of the rock type. Hydrocarbons are often trapped by geologic faults, and so the seismic images are subsequently interpreted to identify rock types which, in conjunction with geologic faults or other phenomenon, would result in an accumulation of fluids in an area in the region of study. However seismic information provides only limited information. Unless the geologic fault has a seal, fluids will not be trapped. Unfortunately, seismic survey data does not contain sufficient information to indicate the presence or absence of a seal. Other information is desirable which, if used in conjunction with seismic images, would improve the probability of finding hydrocarbons.
It is known that the field of so-called prestresses in the earth""s crust is determined by two systems of external forces, namely, gravitational and tectonic forces. The joint action of these forces can result in compaction as well as in unloading of rock masses with respect to their normal gravitational or lithostatic stress condition. The existing theory describing stress fields in the earth""s crust is absolutely correct for elastic and continuous media. At the same time information derived from a number of wells, drilled down to 10 km, points to the presence of fractures in sedimentary and crystalline parts of the earth""s crust. Since there is little reason to expect presence of spatially localized fractures, we can affirm that any real rock unit is bounded by a closed system of fractures and represents a discrete system. Taking into account a well-known concept that any basin represents a tectonically young and consequently active structure, these are all reasons to consider any unit of the sedimentary cover as a part of discrete dynamic system. To date, no one has thought to use information pertaining to stress in the earth""s crust as part of a method for determining the presence of fluid in a subterranean formation. Our invention uses seismic imaging information in conjunction with formation stress information to improve the reliability of seismic exploration for hydrocarbons.
The publication of J. D. Byerlee (xe2x80x9cFriction of Rocksxe2x80x9d, Pure Applied Geophysics, 116 (1978), pp. 615-626) is useful for estimating parameters of stress condition of fractured and discrete rock units. He has found that the maximum differential stress (the difference of main stresses) in the upper part of the earths crust is limited by the shear strength of fractured rocks. Laboratory investigations show that the sliding friction on the surface of fractured rocks exists until the external load reaches the critical value of brittle failure. At the same time the strength of such a discrete rock unit is determined by cohesion of fractures on its surface and does not practically depend on the elastic moduli of the rock, temperature, strain value, and the type of sliding surface. It is determined that the cohesive force xcfx84 is a linear function of normal load "sgr"n (Byerlee""s law):
xcfx84=0.85"sgr"n 3 less than "sgr"n less than 200Mxcfx80axe2x80x83xe2x80x83(1)
or in the terms of main stresses, the limit value of horizontal stresses down to the depths of 5 km is estimated to be
"sgr"1≈5"sgr"3 "sgr"3 less than 110Mxcfx80axe2x80x83xe2x80x83(2)
In other words the main component of stress condition variation in discrete media is associated with the change of vertical load. Insignificant movement in the base of a sedimentary basin (for example, in the crystalline basement) results in considerable change of the horizontal component of stress. The strength limit of a rock unit is rapidly reached, and all the sedimentary cover is set in motion continuously approaching the isostatic equilibrium. Thus, it is reasonable to suggest that real sedimentary formations have a high degree of mobility in vertical direction. As time of stress relaxation characteristic for discrete media is significant, the stress distribution in the basin has distinctive contrasting character and is governed by the principles of block dynamics. The most fundamental theoretical results in the field of elastic wave propagation in prestressed media were derived by M. A. Biot (textbook, Mechanics of Incremental Deformations, New York, 1965). Biot introduced a new type of strain, strain of solid rotation. Its introduction is related to non-hydrostatic character of prestress field in the medium (i.e. horizontal and vertical components of the stress field are not equal to each other). In this case each unit volume of the medium has been rotated through some angle with respect to its initial position in the unstressed medium. As a result additional strains appear in the wave field, and their amplitudes are directly proportional to the difference between horizontal and vertical components of the prestress field ("sgr"1"Hslashed""sgr"3). A number of researchers studied the equations derived by M. A. Biot and came to the conclusion that the influence of initial stresses on traveltime and, especially, amplitude parameters of seismic waves, can be very significant, and the degree of this influence is proportional to the ratio P/xcexc (where P is pressure and xcexc is the shear modulus).
A significant number of publications are related to investigation of the stress condition of the earth with the aid of seismic methods. But practically all these investigations were based on the analysis of traveltime parameters and were reduced to attempts to explain complex distribution of velocity parameters by the influence of lithostatic pressure. In this case the stress condition of the subsurface is taken into account indirectly using theoretically and experimentally derived relationships between velocity and external load. In low-pressure regions the amplitudes and frequencies of the reflected signals completely depend on the density of fractures and the load applied to them. And the earth material (its elastic moduli) significantly influences the amplitude parameters of the signals only at the loads close to the strength limit of discrete media.
What is needed then is a method for improving the ability to discover hydrocarbons in a it st subterranean formation and which makes use of available data, an in particular seismic data which is likely to be collected as part of a exploratory effort in any event.
The presented invention has been developed as a method for seismic data interpretation targeted at discovery of the areas of the accumulations of the fluids. We have discovered a relationship between reflection coefficient and general rock pressure, which is the basis of our invention. It is assumed that the real sedimentary sequence is a discrete medium subjected to inhomogeneous stresses created by two types of the forces, namely gravitational and tectonic forces. As a result of a sum influence of these two forces, the basin, as a discrete system, is found in continuous movement or in the condition of inhomogeneous stresses. In correspondence with this point of view any stratigraphic sequence is found in different stress conditions in lateral direction, and consequently the amplitude and frequency parameters of reflected signals depend on the value of rock pressure acting on given element of a seismic reflector.
The first step of the developed interpretation method consists in tracking one or several target reflections along stacked time sections. A standard tracking procedure and standard 2D or 3D processing flows are used. The pressure gradients are found using picked travel times for tracked seismic horizons and by calculating parameters of Hillbert transformation (instantaneous amplitudes and phases) for each identified signal. The interpretation formula is directed to estimation of relative pressure changes under the assumption that the smoothed values of instantaneous amplitudes and derivatives of instantaneous phases (instantaneous frequency) correspond to general normal or gravitational pressure within all the study area, while all deviations from the average value represent relative estimates of anomalous pressures and correspond to local regions of decompression and compression.
The obtained map of relative pressure gradients is laid over the isochron map of a tracked horizon improved using the instantaneous phase picking. The principle of determining traveltimes more accurately consists in correction for signal frequency variation along the reflecting horizon in correspondence with the derived dependence of the frequency from pressure. The resulting map of relative pressure variation in combination with reflection time map represents the basis for identifying most probable locations of fluid accumulations. Localized region of anomalous unloading coincident with a closed or semi-closed time high is a physically valid area of hydrocarbon and water accumulation.
The information from the resulting map of relative pressure variation in combination with reflection time map can be further combined with classical seismic interpretation techniques to further enhance the prediction of the presence of fluids in a subterranean formation.
While the invention described herein is particularly useful for predicting the presence of fluid accumulations in a subterranean reservoir, the relationship between reflection coefficient and general rock pressure can also be used to predict rock pressure or pressure gradients within a subterranean formation. Such information can be useful in and of itself, for example, in the study and prediction of earthquakes.
The invention further includes a method for predicting the dynamic parameters of fluids in a subterranean reservoir. More specifically, the dynamic parameters of such fluids are determined by determining the relative estimate of the total ground pressure on the attributes of seismic signals related to a reflecting horizon, or any selected interval of the seismic section. This provides for a method for determining the location of the accumulation fluids in a subterranean formation. The method includes the steps of determining a first velocity vector xe2x80x9cVxxe2x80x9d for migration of fluid in a region of interest in the subterranean formation. The first velocity vector includes attributes of speed and direction of flow of fluid in a first direction in the region of interest. The method further includes determining a second velocity vector xe2x80x9cVyxe2x80x9d for migration of fluid in the region of interest. The second velocity vector includes attributes of speed and direction of flow of fluid in a second direction in the region of interest. The velocity vectors are then extrapolated to identify the fluid accumulation location. The first and second velocity vectors are primarily functions of supplementary pressure xe2x80x9cdPxe2x80x9d in the region of interest, the permeability xe2x80x9ccxe2x80x9d of the region of interest, and the viscosity xe2x80x9cuxe2x80x9d of the fluid in the region of interest.
The supplementary pressure can be determined by identifying pressure gradients within the region, the region being characterized by a seismic image of a stacked time section representing horizons within the region. The permeability of the media within the region, and the viscosity of the fluid within the region, can either be determined mathematically or from geological data.
In addition to the methods disclosed and described herein, our invention further includes computer apparatus for implementing the methods.